1. Field of the Invention
The present invention relates to a temperature sensor, and in particular to a temperature sensor using a resonator.
Various temperature sensors have been proposed depending on the applications thereof and have been reduced to practice. For any temperature sensor, a temperature reproducibility, a temperature range, a temperature characteristic, and an accuracy required for the applications are important.
2. Description of the Related Art
FIG. 17 shows a frequency-temperature characteristic of a Surface Acoustic Wave (hereinafter, abbreviated as SAW) resonator receiving attention in various fields for its superior temperature reproducibility. This characteristic is shown by a quadratic curve in which a frequency “f” exhibits a maximum value (peak) at a temperature T1.
FIG. 18 shows a prior art arrangement of a temperature sensor using a SAW resonator. In this example, an oscillator circuit 20 is arranged so as to connect a SAW resonator 21 as an oscillator to an amplifier 22 and a temperature sensor 10 measuring the oscillation frequency of the oscillator circuit 20 by a reference frequency is composed.
A counter 31 counts an output of the oscillator circuit 20. A reference frequency oscillator circuit 40 produces a latch signal 92 for latching, in a register 41, a value of the counter 31 at a fixed time interval (e.g. 1 sec.) based on an internal reference frequency, and a reset signal 91 provided to the counter 31 after the latching.
A frequency/temperature converter 50 includes a table storing a relationship between the frequency “f” and the temperature “T” shown in FIG. 17 preliminarily obtained by experiments or the like, and outputs, referring to this table, a temperature signal 90 obtained by converting a frequency value from the register 41 into a temperature value.
In such a temperature sensor 10, an accurate frequency reference signal corresponding to a required temperature accuracy becomes necessary, so that the circuit price is increased as the accuracy increases.
As for a measurable temperature range at this time, it is supposed that either a temperature range higher than a temperature T1 or a temperature range lower than the temperature T1 is preliminarily selected lest two temperatures correspond to the same frequency. Also, since a gradient around the temperature T1 is small, the temperature greatly changes even for a little frequency change and the measurement accuracy declines. For this reason, it is common to remove the range around the temperature T1 from available ranges.
Namely, when the frequency-temperature characteristic is a quadratic curve, a cubic curve, or more, there is a possibility that two or more temperatures correspond to the same frequency. Therefore, the measurable temperature range is disadvantageously limited by the frequency-temperature characteristic of the SAW resonator.
It is to be noted that the above-mentioned problems are common to the temperature sensors using not only the SAW resonator but also a general oscillator.